1. Field of the Invention
Embodiments disclosed herein relate generally to components of wellbore fluids (muds). More specifically, embodiments relate to fluid loss additives used in oil-based muds.
2. Background Art
When drilling or completing wells in earth formations, various fluids typically are used in the well for a variety of reasons. Common uses for well fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in (i.e., drilling in a targeted petroliferous formation), transportation of “cuttings” (pieces of formation dislodged by the cutting action of the teeth on a drill bit) to the surface, controlling formation fluid pressure to prevent blowouts, maintaining well stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, transmitting hydraulic horsepower to the drill bit, fluid used for emplacing a packer, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
In most rotary drilling procedures the drilling fluid takes the form of a “mud,” i.e., a liquid having solids suspended therein. The solids function to impart desired rheological properties to the drilling fluid and also to increase the density thereof in order to provide a suitable hydrostatic pressure at the bottom of the well. The drilling mud may be either a water-based or an oil-based mud.
Many types of fluids have been used in well bores particularly in connection with the drilling of oil and gas wells. The selection of an oil-based well bore fluid involves a careful balance of both the good and bad characteristics of such fluids in a particular application. The primary benefits of selecting an oil-based drilling fluid include: superior hole stability, especially in shale formations; formation of a thinner filter cake than the filter cake achieved with a water based mud; excellent lubrication of the drilling string and downhole tools; penetration of salt beds without sloughing or enlargement of the hole as well as other benefits that should be known to one of skill in the art. An especially beneficial property of oil-based muds is their excellent lubrication qualities. These lubrication properties permit the drilling of wells having a significant vertical deviation, as is typical of off-shore or deep water drilling operations or when a horizontal well is desired. In such highly deviated holes, torque and drag on the drill string are a significant problem because the drill pipe lies against the low side of the hole, and the risk of pipe sticking is high when water based muds are used. In contrast oil-based muds provide a thin, slick filter cake which helps to prevent pipe sticking and thus the use of the oil-based mud can be justified.
Drilling fluids are generally characterized as thixotropic fluid systems. That is, they exhibit low viscosity when sheared, such as when in circulation (as occurs during pumping or contact with the moving drilling bit). However, when the shearing action is halted, the fluid should be capable of suspending the solids it contains to prevent gravity separation. In addition, when the drilling fluid is under shear conditions and a free-flowing near-liquid, it must retain a sufficiently high enough viscosity to carry all unwanted particulate matter from the bottom of the well bore to the surface. The drilling fluid formulation should also allow the cuttings and other unwanted particulate material to be removed or otherwise settle out from the liquid fraction.
There is an increasing need for drilling fluids having the rheological profiles that enable wells to be drilled more easily. Drilling fluids having tailored rheological properties ensure that cuttings are removed from the wellbore as efficiently and effectively as possible to avoid the formation of cuttings beds in the well which can cause the drill string to become stuck, among other issues. There is also the need from a drilling fluid hydraulics perspective (equivalent circulating density) to reduce the pressures required to circulate the fluid, reducing the exposure of the formation to excessive forces that can fracture the formation causing the fluid, and possibly the well, to be lost. In addition, an enhanced profile is necessary to prevent settlement or sag of the weighting agent in the fluid, if this occurs it can lead to an uneven density profile within the circulating fluid system which can result in well control (gas/fluid influx) and wellbore stability problems (caving/fractures).
To obtain the fluid characteristics required to meet these challenges the fluid must be easy to pump, so it requires the minimum amount of pressure to force it through restrictions in the circulating fluid system, such as bit nozzles or down-hole tools. In other words the fluid must have the lowest possible viscosity under high shear conditions. Conversely, in zones of the well where the area for fluid flow is large and the velocity of the fluid is slow or where there are low shear conditions, the viscosity of the fluid needs to be as high as possible in order to suspend and transport the drilled cuttings. This also applies to the periods when the fluid is left static in the hole, where both cuttings and weighting materials need to be kept suspended to prevent settlement. However, it should also be noted that the viscosity of the fluid should not continue to increase under static conditions to unacceptable levels. Otherwise when the fluid needs to be circulated again this can lead to excessive pressures that can fracture the formation or lead to lost time if the force required to regain a fully circulating fluid system is beyond the limits of the pumps.
Further, it is also important that a drilling possess the ability to resist filtration. To prevent formation fluids from entering the wellbore hole, the hydrostatic pressure of the mud column must be greater than the pressure of the fluids in the pores of the formation. As a result, there is a tendency for the liquid portion of wellbore fluids to invade the permeable wellbore formations, which is referred to as the filtrate. As a result, the mud solids are filtered out onto the walls of the wellbore hole, forming a coating or cake. Thus, the muds must be formulated so as to keep the deposited cake permeability as low as possible in order to maintain a stable borehole and minimize filtrate invasion of, and damage to, potentially productive strata and horizons in the production formation. The loss of filtrate to the formation is referred to as fluid loss.
In addressing issues surrounding fluid loss control, one must choose additives that are compatible with the solvent system of the mud. Oil-based muds frequently use invert emulsions with a mixture of oil and water in the presence of surfactants. Fluid loss control materials typically used in drilling muds include plant tannins, such as those found in the wood of quebracho, chestnut, oak and urunday, and in the bark of wattle, mangrove, oak, eucalyptus, hemlock, pine, larch, and willow trees. Other materials include starches (e.g. corn, potato), starch derivatives, water soluble cellulose derivatives, humates, polyphosphates or phosphate-containing materials, lignite materials, lignosulfonates and synthetic polymers (especially swellable polymers and gels).
Many of these polymers and condensed-tannin materials are either water soluble or at least hydrophilic and thus benefit from chemical modification to impart organophilic character when used in conjunction with oil/invert emulsion-based muds. For example, U.S. Pat. No. 4,421,655 discloses the use of humates modified with amido amines in the presence of polyvalent metal ion. U.S. Pat. No. 4,710,586 discloses the use of alkyl quaternary ammonium salts reacted with quebracho for use as fluid loss additives. Another consideration is the use of materials that are environmentally compatible
The materials that affect the rheological profile of oil-based muds may include both viscosifiers and fluid loss additives. Accordingly, there exists a continuing need for improving these drilling materials and wellbore fluid formulations.